Glucose (Glc), a
monosaccharide (or simple
sugar), is one of the most important
carbohydrates. The cell uses it as a source of energy and metabolic intermediate. Glucose is one of the main products of
photosynthesis and starts
cellular respiration. The natural form (
D-glucose) is also referred to as
dextrose, especially in the food industry. This article deals with the D-form of glucose (The mirror-image of the molecule is called
L-glucose. See also
Isomers-section below)
Structure
Glucose contains six
carbon atoms and an
aldehyde group and is therefore referred to as an aldo
hexose. The glucose molecule can exist in an open-chain (acyclic) and ring (cyclic) form, the latter being the result of an intramolecular reaction between the aldehyde C atom and the C-5
hydroxyl group to form an intramolecular
hemiacetal. In water solution both forms are in equilibrium, and at
pH 7 the cyclic one is the predominant. As the ring contains five carbon atoms and one oxygen atom, which resembles the structure of
pyran, the cyclic form of glucose is also referred to as glucopyranose. In this ring, each carbon is linked to an hydroxyl side group with the exception of the fifth atom, which links to a sixth carbon atom outside the ring, forming a CH
2OH group.
Isomers
Glucose has 4 optic centers which means that in theory glucose can have (4²-1) = 15
optical stereoisomers. Only 7 of these are found in living organisms, and of these
galactose (Gal) and
mannose (Man) are the most important. These eight isomers (including glucose itself) are all
diastereoisomers in relation to each other and all belong to the
D-series.
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